Natural Selection
Natural selection is the process in which some environmental factors determines which individuals will survive. We say individuals are 'selected' from the population, or that they undergo selection pressure. If the individual has beneficial characteristics, it will be at an advantage. It will be selected to survive and pass on its beneficial characteristics. However if it does not have enough beneficial characteristics the indiviudal will not be 'selected' to survive. It will struggle and die, so will not pass on its characteristics.
Examples of factors in the environment that act as selective forces:

availability of suitable food - if an individual is adapted to eat the available food it is an advantage
diseases - if an organisms can survive a disease it has a selective advantage
predators - an individual adapted to avoid being seen and eaten, or to escape, has a selective advantage
physical and chemical factors - if an organism can survive, for example, growing in a very shady place or in a desert, or living in a place with extremely cold winters and very hot summers, it has a selective advantage

New Species
The formation of a new species from a pre-existing one is called speciation.

How long does speciation take?
Forming two closely related species from one does not occur suddenly. It is a long, slow accumulation of changes. These eventually mean that individuals can no longer interbreed freely to produce viable offspring. It is likely to take many generations. That is not to say it cannot happen overnight. Bacteria and single-celled organisms can pass through several generations in a few hours. This may be sufficient to allow speciation to occur.

How does speciation occur?
In order to form a new species from one original group of organisms, there must be some reproductive barrier. This means that some organisms are unable to breed with others in the group. Variations or changes that provide a benefit spread down the generations in a population through a reproductive. If changes occur in part of the group, but cannot spread to the whole group, then only part of the group will benefit. A collection of small changes that cannot pass to the whole group means that some members will become different from the others. They may become so different that they can no longer interbreed.

Reproductive Barriers
A reproductive barrier is any factor that prevents effective reproduction between members of species.

Geographical separation will prevent effective interbreeding between the individuals of two populations. Different groups of the same species living on different islands will be unlikely to interbreed freely. So speciation is likely to occur. This is what allowed the evolution of a new species in the Galapagos Islands. This is known as allopatric speciation.
Sometimes a reproductive barrier may arise within the population. This may be due to a biochemical change that prevents fertilisation. It may be due to a behavioural change. Or it may be due to a physical change, where the sexual organs of two groups of individuals are no longer compatible and they cannot mate. Any change that prevents one member of the population breeding with another can act as a reproductive barrier. This is known as sympatric speciation.

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Undergraduate Biology (Biodiversity and Evolution) Notiz am Natural Selection, erstellt von siobhan.quirk am 27/05/2013.